In many communication systems, data is often converted into a passband signal, e.g., centered around a carrier frequency, before transmission. One reason for converting the original signal into a passband signal is that the conversion allows multiple channels of data to be transferred over a single transmission medium, e.g., by using several different carrier signals. A common example of this is radio broadcasts.
Since the transmitted signal is a passband signal, the received signal is also passband. In many systems, the passband signal is first converted to its baseband, i.e., is centered around zero frequency as opposed to the carrier frequency, before further signal processing takes place. The generation of the baseband signal is in many cases done with analog devices before any analog-to-digital conversion takes place. The baseband signal normally comprises an in-phase (I) component and a quadrature (Q) component.
The baseband signal may be any one of several different signal formats which are possible. Many transmitted signals are used to transmit values known as symbols. Various symbol transmission systems are designed so that symbols will be distributed in an I/Q plane relatively symmetrically about the origin over a period of time.
The I and Q components of a baseband signal are often processed separately, e.g., in parallel. As part of the steps to obtaining a baseband signal, the passband signal is copied and multiplied by a cos (2πfct) signal to generate the I component and the same passband signal is copied and multiplied by a sin (2πfct) signal to generate the Q component. In principle, the in-phase cos (2πfct) and quadrature sin (2πfct) components should have exactly π/2 phase shift and the same amplitude. However, in reality it is very difficult and costly to achieve a highly accurate π/2 phase shift and equal amplitude using analog devices. Consequently, the resultant in-phase and quadrature components generally have imbalance in amplitude and/or phase, i.e., I/Q imbalance, which causes signal quality degradation in the subsequent receiver signal processing.
FIG. 1 illustrates an exemplary 16-QAM (quadrature amplitude modulation) constellation 10, which is an example of a modulation scheme used to transmit data. Each symbol in the constellation is denoted by an “x”. In known 16-QAM the permissible nominal symbol values for both the x and y coordinates is (±1, ±3) with the nominal squared magnitude being approximately 2, 10 and 18. The rings are included in FIG. 1 to show how the symbols are distributed symmetrically around the original of the I/Q plane. As a result of phase imbalance, received symbols might appear to be distributed along an oval centered at the origin as opposed to around a circular ring centered at the origin. Amplitude imbalance may case the radius of the rings on which the symbols are located to deviate from the ring's intended radius. Such errors can complicate the process of accurate symbol interpretation.
In-Phase and Quadrature phase (I/Q) signal imbalance is a well-known problem in the receiver design of many communication systems. Therefore, many I/Q imbalance compensation devices are known in the art. Unfortunately some of these devices can be very complex in their design. Complex designs are often harder to implement in hardware, take more physical space to implement and have higher processing overhead than simple designs. Many known I/Q imbalance compensation devices only work with a particular type of received signal. Such devices use the specific structure and/or the nature of the received signal to compensate for I/Q imbalance. Unfortunately, those types of compensation devices are often limited in utility to the received signal for which they were designed. Using such devices for other types of received signals may cause more I/Q imbalance rather than compensate for it.
Accordingly, there is a need for new and improved methods and apparatus that can be used to compensate for, reduce, and/or eliminate I/Q imbalance. In addition, the methods and apparatus should be relatively independent of the received signal's structure, thereby making the methods and apparatus applicable to a greater diversity of communication systems than some of the known designs.